1. Field
Example embodiments relate to a semiconductor memory device, and more particularly, to a vertical string phase change random access memory device.
2. Description of the Related Art
Non-volatile memory devices include phase change random access memory (PRAM), ferroelectric random access memory (FeRAM), and magnetic random access memory (MRAM). A phase change random access memory device uses a phase change material in a data storage layer that, for example, changes electrical resistance depending on whether the phase change material is in an amorphous or crystalline phase. Examples of conventional phase change materials include those based on Ge—Sb—Te, which exhibit a higher resistance in the amorphous phase than in the crystalline phase.
The phase transformation between an amorphous phase and a crystalline phase is caused by current generated Joule heating. For example, when a voltage is applied to a phase change material for a relatively short period of time, a portion of the phase change material is heated above a temperature in which the material remains crystalline (e.g., above its crystallization temperature) and the material transforms into an amorphous phase. The material will retain the amorphous phase if it is rapidly cooled. To transform a material from an amorphous phase to a crystalline phase, a voltage is applied to the phase change material for a relatively long period of time in which the material re-crystallizes to a crystalline phase.
In the conventional art, phase change memory devices have a single set and reset state. The phase transformation from an amorphous phase to a crystalline phase is defined as a ‘set’ operation and the voltage applied to the phase change material during the set operation is referred to as a ‘set voltage’. The phase transformation from a crystalline phase to an amorphous phase is defined as a ‘reset’ operation, and the voltage applied to the phase change material during the reset operation is referred to as a ‘reset voltage’. Accordingly, conventional phase change memory devices may only store one binary-bit of information.